Semiconductor devices or integrated circuits which are provided in a flip-chip structure have solder balls for making vertical contact between the integrated circuit and a printed circuit board or a further integrated circuit in a stack. The solder ball is arranged on a connection device or a pad, e.g. on a chip, which is connected to the active section of the chip by metal lines or interconnects which form a rewiring device. The connection device or pad and the layers are often fabricated from a stack of plated metals, such as copper, nickel and a gold covering layer.
All solder ball arrangements or packages require a type of solder stop around the ball. This is required since the solder has a tendency to wet not just the soldering pad itself but also the adjoining rewiring device. Without solder stop structures, the solder would flow away during the solder reliquification or reflow. It is customary for a solder stop resist or resin to be applied to the rewiring, preventing the solder from flowing away.
If the rewiring layout is two-dimensional, the resist is normally applied, in particular spun on, and then patterned by means of a photographic technique. Alternatively, the solder stop material can be applied in a printing process if the feature sizes are large enough. In the case of three-dimensional structures, application is much more difficult. The adhesion or bonding of these organic layers to gold is not very good, which may result in the solder running under the resist and wetting covered sections of the metal lines or interconnects. Furthermore, resists which can be exposed by photographic techniques and have the required temperature stability are expensive.